1. Field of the Invention
This invention relates to a novel organic compound and a liquid crystal composition containing the same. More particularly it relates to an organic compound having an optically active group and a liquid crystal composition containing the same.
2. Description of the Related Art
As to liquid crystal display elements, their use applications have been rapidly broadened due to improvements in circuit, driving mode and cell production technique and particularly due to improvement in characteristics of liquid crystal compositions filled in the elements. However, liquid crystal display elements still have a number of drawbacks to be overcome such as narrow angle of view, inferior contrast, low response rate, still small display capacity, reduction in the display quality due to ambient temperature change, etc. Among these, the reduction in the display quality due to ambient temperature change is originated from the temperature change in the threshold voltage Vth.
As a means usually employed in recent years, there is a method of adding a slight quantity of an optically active substance to a liquid crystal composition to suppress the reverse twist of liquid crystal molecules and thereby afford a right-turn or left-turn helical structure to the liquid crystal molecules so that a display quality is retained. For example, the method can have an advantageous effect upon the cases of TN (Twisted nematic) mode display element, recently proposed SBE (supertwisted birefringence effect) mode display element, etc.
However, if the twistability of an optically active substance added as a dopant is too low, it is necessary to add the dopant in a relatively high concentration in order to obtain a required pitch; hence this apparently has a disadvantageous effect upon other parameters of the liquid crystal composition. Thus, an optically active substance has been awaited, which has a high twistability i.e. a capability of exhibiting a shorter pitch when the substance is added in the same quantity to liquid crystals.
For example, when CB-15 (tradename of product made by BDH Company) or compounds recently disclosed in Japanese patent application laid-open Nos. Sho 62-81354/1987 and Sho 62-81355/1987 are added only in 1% by weight to nematic liquid crystal compositions, these compounds have a capability of exhibiting a pitch of about 10 .mu.m; hence they can be regarded as practically usable optically active substances.
However, general known optically active substances including the above-mentioned compounds increase the pitch of liquid crystal substances with rise of temperature by the addition thereof; thus they often have an undesirable effect upon the pitch. For example, in the case of SBE mode, the intrinsic pitch P of liquid crystal compositions varies with temperature change, whereby the ratio of the intrinsic pitch (P) of the liquid crystal compositions to the cell thickness (d) of the display element (P/d) varies accordingly. The P/d value is usually 2 or less, but when it exceeds 2 due to the temperature change, the 270.degree. twist changes to the 90.degree. twist.
Further, in the aspect of improvement for enhancing the display capacity, it is necessary to improve the steepness of change in the transmittance in the case where a voltage is going to be impressed to a display element. G. Bauer and W. Fehlenback reported a calculation result that the steepness was notably improved by making the twist 270.degree., in the 15th Freiburg liquid crystal sympodium (1985), and in this case, too, it is necessary to be free from change in the intrinsic pitch due to temperature.
As a means for solving this problem, an optically active compound having negative temperature characteristics i.e. a substance for reducing its intrinsic pitch with temperature rise has recently been found and by blending a suitable quantity of this compound with an optically active substance having positive temperature characteristics, a composition having its intrinsic pitch unchanged due to temperature has been obtained (Emoto et al., Japanese patent application No. Sho 61-179194/1986).
However, any of currently reported substances having negative temperature characteristics have a low twistability; hence it is necessary to add such substances in a considerably high concentration in order to obtain a required pitch so that it is considered that they have various bad influences. Thus, in order to solve these problems, a substance having a short pitch and yet negative temperature characteristics has been desired.
The present inventors have made extensive research in order to develop an optically active compound having characteristics required for realizing a liquid crystal composition having suitable characteristics i.e. a negative temperature characteristic and a high twistability, and as result have achieved the present invention.